It is known that these tin oxide coatings are useful for imparting to flat glass a heat reflective low emissivity surface. Such coating glass finds particular use in energy efficient windows as well as oven and refrigerator doors, sodium vapor lamps, incandescent and fluorescent lights.
These coatings are also useful as resistor heating elements for de-icing windows in airplanes or automobiles, as antistatic coatings on instrument panels, as electrodes for many types of optical-electronic devices and as protective coatings for bottles, optical fibers and carbon fibers.
Although the glass industry has sought to utilize the properties of tin oxide films to economically produce coated glass for energy efficient windows, success has been limited to low to medium performance products and high performance-high cost products. The desire, however, is for a high performance, economical, on-line applied tin oxide coating which provides a sheet resistance of 40 ohm/sq. at 250 nm, with visible transparency and infrared reflectivity approaching 90%.
Tin oxides films have been applied to glass by many techniques and most require an organic or inorganic tin precursor combined either chemically or physically with a material called a dopant. The dopant is responsible for imparting to the film the high conductivity and infrared reflectivity desired. Numerous materials have been used as dopants but the most effective for tin oxide contains the element fluorine, either bonded to the tin precursor directly, bonded to a group attached to tin or as part of a separate compound.
Doped stannic chloride and stannous chloride solutions have been sprayed directly onto hot glass to produce conductive tin oxide films. These films are often hazy in appearance, non-uniform in thickness, non-reproducible in application and have undesirably high sheet resistance (50 ohm/sq.). This is the result of using a corrosive, easily hydrolyzable chemical and an application method that is influenced by many variables. The presence of large volumes of solvent further complicates solution deposition processes. Some of these problems can be overcome when stannic chloride is applied by vapor deposition (CVD). CVD requires that the chemicals have sufficient vapor pressure below their decomposition temperature to deposit sufficient material. No solvents are used and uniform gas mixtures are readily attained and easily transported. U.S. Pat. No. 4,387,134 teaches that tin oxide films with sheet resistances of 1-10 ohm/sq were produced from a complex combination of vaporized water, methanol, HF, stannic chloride and H.sub.2 /N.sub.2 gases. However, the films were at least 5000 A thick, had transparencies as low as 60% and IR reflectivities as low as 50%, values totally unacceptable for residential window glass.
Gordon teaches in U.S. Pat. No. 4,265,974 that the CVD of doped tetramethyltin produces tin oxide films with good optical and electrical properties. However, tetra methyltins' toxicity and the even greater toxicity of the by-products, combined with low rate of deposition on sheet glass, are serious disadvantages.
Another process which avoids the problems associated with solution spray, is that of fine powder deposition. U.S. Pat. No. 4,325,988 teaches that a very fine dust of a material with a particle size of less than 10 microns can be mixed with a carrier gas and transported to a hot glass surface where it decomposes to form an oxide film. Dibutyltin difluoride has been deposited in this way and a clear uniform film of SnO.sub.2 can be produced with a sheet resistance of 20 ohms/sq. However, the solid must have a uniform size distribution and must be about to 1-2 microns in diameter. Producing material with a particular uniform size distribution and subjecting it to an additional grinding step is time-consuming, difficult and adds cost penalties to this deposition technique. Moreover, the design of application and recovery equipment is somewhat complex and the rate of deposition is slow.
Various vacuum techniques such as ion sputtering have been developed which produce clean, uniform tin oxide films with good properties but the cost of equipment is high, the process is a batch operation and the finished coated surface must be protected from the environment because of poor film adhesion to the glass.
U.S. Pat. No. 4,293,594 discloses a method for forming a highly conductive transparent coating on a substrate forming a vapor of mixed organic tin halide compound and an organic fluoride compound in an oxygen containing carrier gas and contacting a heated vitreous substrate with the organic tin halide containing carrier gas. The gas phase is heated to a temperature of about 190.degree. C. to about 350.degree. C. and the vitreous substitute is heated to a temperature greater than 350.degree. C., e.g., 350.degree. C. to 650.degree. C.
While the U.S. Pat. No. 4,293,594 patent does not teach a generic class of compounds suitable for use as the organic tin halide precursors for the tin oxide film formed, the compounds disclosed by the patent can be described by the formula EQU R.sub.a SnX.sub.b
wherein R is methyl, ethyl, butyl or phenyl; X is halogen, preferably chlorine; and a and b are integers selected so that a+b=4. Illustrative of the compounds disclosed are dimethyltin dichloride, triethyltin chloride, butyltin trichloride, dibutyltin dichloride and triphenyltin fluoride.
Compounds disclosed as being suitable dopants for the tin oxide film include dimethyl tin difluoride, ammonium acid fluoride and phosphorous pentoxide. However, many of the compounds disclosed have such low vapor pressures or high melting points that they could not be vaporized in accordance with the teachings of the patent.
An earlier patent, U.S. Pat. No. 3,677,814, disclosed the use of fluoride containing tin compound, where a fluoride atom is attached to the tin, to simultaneously form the tin oxide film and dope it to be electrically conductive. Alkyl and aryl tin compounds of the formula RxSnF.sub.4-x wherein x is an interger of 1 to 3 and R is alkyl or aryl are alleged to be useful in the practice of the invention. The tin oxide films are prepared by spraying a solution of the organotin fluoride in a hot glass surface, e.g., 1300.degree. F. Only dibutyltin difluoride is illustrated. Large quantities of solvent are required to solubilize this poorly soluble material.
U.S. Pat. Nos. 4,389,238 and 4,130,673 disclose apparatus and methods for coating glass bottles with tin oxide in order to reduce the coefficient of friction of the glass, thereby reducing breakage. The latter patent teaches the use of butyltin trichloride as the precursor for the tin oxide which is formed by spraying the hot bottles with butyltin trichloride in an air stream from an atomizer orifice.
Japanese KoKai No. 75,61,415 discloses the combination of dibutyltin diacetate, ethythdifluoroacetate dissolved in isopropyl alcohol as being useful for the preparation of doped SnO.sub.2 films. However, these solutions give doped tin oxide films with resistivities about one order of magnitude higher than those achieved by the process of the invention of this disclosure. U.S. Pat. No. 3,949,146 discloses as useful tin oxide precursors compounds of the formula R.sub.2 Sn(OOCR).sub.2 or (R.sub.3 Sn).sub.2 O wherein R is a lower alkyl, and inferentially teaches that the presence of water vapor in the carrier gas stream improves film conductivity. However, the disclosure makes no mention of the importance of dopant selection or water vapor levels required. The disclosure, when followed, will result in a product having unacceptably low conductance.
European Patent Office Publication No. 0 112 780, published July 4, 1984 discloses the use of butyltin trichloride to form conductive films of tin oxide using dichlorodifluoromethane as the dopant.
The selection of particular dopants for stannic oxide films and process parameters for the production of such electrically conductive films forms the basis of this invention.